1. Field of the Invention
The present invention relates to an apparatus for connecting secondary battery cells in series and a method for controlling secondary battery cells in series so as to properly control them.
2. Description of the Related Art
In recent years, as portable telephone terminal units, audio reproducing units, note type personal computer have become common, rechargeable secondary battery cells have become important. In addition, as such portable units have provided high performances, their power consumptions increase. Thus, there have been needs for secondary battery cells that have higher power outputs for longer time than before. Nevertheless, the secondary battery cells structurally have their maximum voltages. Consequently, when a higher voltage is required, several to several ten secondary battery cells are connected in series.
In recent years, lithium type battery cells such as lithium ion battery cells and polymer lithium battery cells have become the mainstream of secondary battery cells.
When secondary battery cells that are connected in series are used, as the power is consumed, the battery cell capacities are worn out. As a result, output voltages of the connected battery cells may become different and the battery cells may become unbalanced. When battery cells are connected in series, the positive electrode of one battery cell is connected to the negative electrode of the next battery cell. However, when the battery cells become unbalanced, a reverse charging takes place from one battery cell that has a higher voltage to another battery cell that has a lower voltage. In other words, a charging takes place with opposite electrodes. Thus, if battery cells are kept in the unbalanced state, the reverse charging may endanger the battery cell that has the lower voltage.
In addition, if secondary battery cells that are connected in series are charged in the unbalanced state, a trouble will take place. Now, it is assumed that a battery pack that contains two secondary battery cells having a capacity of 4.2 V each connected in series is charged. In addition, it is assumed that the output voltage of one secondary battery cells drops to 4.0 V. In the case, the output voltage of the other secondary battery cell is still 4.2 V. Thus, the output voltage of the battery pack is 8.2 V.
A charger charges the battery pack for 0.2 V so that the output voltage thereof becomes 8.4 V. As a result, the two secondary battery cells are charged for 0.1 V each. In other words, the secondary battery cell whose output voltage has dropped and the secondary battery cell whose output voltage has not dropped are charged at 4.1 V and 4.3 V, respectively. Thus, since the other battery cell is overcharged against the regular capacity of 4.2 V, it may become endangered.
To prevent the battery cells from becoming unbalanced, it is necessary to detect the voltages of the secondary battery cells that are connected in series and keep the secondary battery cells balanced corresponding to the detected voltages. As one method, the voltages of secondary battery cells that are connected in series can be separately detected. As another method, the voltages of secondary battery cells are measured at more than two points and calculated by a calculation.
FIG. 1 shows an example of a structure of which the voltages of secondary battery cells that are connected in series are separately detected and the battery cells are balanced. Referring to FIG. 1, secondary battery cells E100 and E101 are connected in series. The voltages of those secondary battery cells E100 and E101 are detected by detecting circuits 100 and 101, respectively. The detected results are supplied to a controlling circuit 103. Discharging circuits 104 and 105 discharge the secondary battery cells E100 and E101 under the control of the controlling circuit 103. A power source of the controlling circuit 103 is obtained by a voltage down circuit 102. The voltage down circuit 102 drops the output voltages of the secondary battery cells E100 and E101, which are connected in series, to predetermined voltages. The voltage down circuit 102 also stabilizes the voltages.
In the structure shown in FIG. 1, the voltages of the secondary battery cells E100 and E101 are detected by the detecting circuits 100 and 101, respectively. The secondary battery cells E100 and E101 are discharged by the discharging circuits 104 and 105, respectively, under the control of the controlling circuit 103 corresponding to the detected results. As a result, the secondary battery cells E100 and E101 are balanced.
In the method, while the secondary battery cells E100 and E101 are being discharged, they are balanced. Thus, it takes a long time until the secondary battery cells become balanced.
In addition, since the voltage down circuit 102 drops the output voltage of which the secondary battery cells E100 and E101 are connected in series to a predetermined voltage and supplies the dropped voltage to the controlling circuit 103, a large loss takes place.
Moreover, there is also a problem about detected voltages of the secondary battery cells E100 and E101. FIG. 2 is an enlarged view showing the voltage detecting portions of FIG. 1. In the conventional method, two voltage detecting circuits namely the detecting circuits 100 and 101 are used. Thus, the detection characteristics of the detecting circuits 100 and 101 deviate. As a result, the discharging of the secondary battery cells E100 and E101 cannot be accurately controlled by the controlling circuit 103.
In addition, in the method, when the voltage of the secondary battery cell E100 is detected, the detected portion departs from the ground potential. Thus, in the state that the voltage of the secondary battery cell E101 is added, the voltage of the secondary battery cell E100 is detected. As a result, the detected result contains an error.
To prevent such an error, it is possible to measure the output voltages of the secondary battery cells E100 and E101 with a single detecting circuit. FIG. 3 shows a structure in the case that a single detecting circuit is used. When the voltage of the secondary battery cell E100 is detected, SW100 and SW102 are turned on. SW101 and SW103 are turned off. When the voltage of the secondary battery cell E101 is detected, SW101 and SW103 are turned on. SW100 and SW102 are turned off. When those switches are controlled in such a manner, the single detecting circuit 102 can detect the voltages of the secondary battery cells E100 and E101.
However, like the structure shown in FIG. 2, in the method shown in FIG. 3, when the voltage of the secondary battery cell E100 is detected, the detected portion departs from the ground potential. In the state that the voltage of the secondary battery cell E101 is added, the voltage of the secondary battery cell E100 is detected. As a result, the detected result contains an error.